Differential amplifiers are used for many different applications. Differential amplifiers are used for signal transmission, such as differential analog crosspoint switches. A differential amplifier generates an output voltage that is proportional to the difference between voltages present on a pair of inputs. Often, the output can be a differential signal, for example a differential voltage signal or a differential current signal. In many applications, the differential input signal is a time varying signal and the differential amplifier circuit must generate a corresponding time varying output rapidly.
The performance of differential amplifiers often is discussed in the context of two different properties. The first property being the differential output of the differential amplifier. Generally, it is desirable to have the differential output to be an accurate representation of the differences between the signals applied to the inputs, and to have the difference amplified by a stable gain. It is further desirable for the response of the differential amplifier to be rapid with respect to fluctuations in the input signals. The second property being a common mode in which it is desirable to maintain the operation of the circuit devices within their linear operating range. This allows the average of differential output signals to remain at some predetermined level with relatively low fluctuation.
A goal of the present invention is to maintain the high speed characteristics of a differential amplifier while providing stability from a common-mode loop by using dominant-pole compensation. This is accomplished by multiplying the current in a compensation circuit from the common-mode feedback loop without multiplying the current from the differential feedback loop.